19.1 Heart Anatomy

The heart, a fist-sized powerhouse, sits in the chest between the lungs. It's a complex pump with four chambers, valves, and layers of tissue. The heart's structure allows it to efficiently move blood through two circulatory loops.

Blood flows through the heart in a specific sequence, powered by coordinated contractions. The right side handles deoxygenated blood, while the left deals with oxygenated blood. Coronary vessels nourish the heart itself, ensuring it can keep up its vital work.

Heart Anatomy and Function

Position of heart in thorax

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• Located in the mediastinum, the central compartment of the thoracic cavity
  • Roughly the size of a closed fist (human hand)
  • Situated between the lungs (left and right)
  • Two-thirds of the mass is to the left of the body's midline
• Protected by the sternum anteriorly, the vertebrae posteriorly, and the rib cage (thoracic cage)
• Rests on the diaphragm inferiorly

External and internal heart structures

• Apex: pointed tip of the heart, situated inferiorly and pointing to the left
• Base: opposite the apex, situated superiorly and posteriorly
• Coronary sulcus: external groove marking the separation between the atria and ventricles
• Interatrial septum: internal wall separating the right and left atria
• Interventricular septum: internal wall separating the right and left ventricles
  • Muscular part: thick, lower portion of the interventricular septum
  • Membranous part: thin, upper portion of the interventricular septum
• Papillary muscles: cone-shaped projections in the ventricles that attach to chordae tendineae (heart strings)
• Chordae tendineae: fibrous strings that connect papillary muscles to the atrioventricular valves (mitral and tricuspid)

Layers of cardiac tissue

• Epicardium: outer layer of the heart wall, composed of mesothelium and connective tissue (serous pericardium)
• Myocardium: middle layer of the heart wall, composed of cardiac muscle tissue
  • Responsible for the heart's contraction and pumping action
• Endocardium: inner layer of the heart wall, composed of endothelial cells and connective tissue
  • Continuous with the endothelial lining of blood vessels (arteries and veins)
• Pericardium: protective sac surrounding the heart, consisting of fibrous and serous layers

Heart structure for pumping

• Atria: upper chambers of the heart that receive blood from the veins
  • Right atrium receives deoxygenated blood from the systemic circulation (body)
  • Left atrium receives oxygenated blood from the pulmonary circulation (lungs)
• Ventricles: lower chambers of the heart that pump blood out to the arteries
  • Right ventricle pumps deoxygenated blood to the pulmonary circulation (lungs)
  • Left ventricle pumps oxygenated blood to the systemic circulation (body)
• Valves: ensure unidirectional blood flow through the heart
  • Atrioventricular (AV) valves: located between the atria and ventricles
    • Tricuspid valve: between the right atrium and right ventricle
    • Mitral (bicuspid) valve: between the left atrium and left ventricle
  • Semilunar valves: located at the base of the pulmonary trunk and aorta
    • Pulmonary valve: at the base of the pulmonary trunk
    • Aortic valve: at the base of the aorta

Systemic vs pulmonary circulation

• Systemic circulation: carries oxygenated blood from the left ventricle to the body tissues and returns deoxygenated blood to the right atrium
  • Aorta: largest artery in the body, carries oxygenated blood from the left ventricle (to organs and tissues)
  • Superior and inferior vena cava: large veins that return deoxygenated blood to the right atrium (from upper and lower body)
• Pulmonary circulation: carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium
  • Pulmonary trunk: large artery that carries deoxygenated blood from the right ventricle to the lungs (for oxygenation)
  • Pulmonary veins: four veins that return oxygenated blood from the lungs to the left atrium

Blood vessels of coronary circulation

• Coronary arteries: supply oxygenated blood to the heart muscle (myocardium)
  • Left coronary artery: arises from the left aortic sinus
    • Left anterior descending (LAD) artery: supplies the anterior wall and interventricular septum
    • Circumflex artery: supplies the left lateral and posterior walls
  • Right coronary artery: arises from the right aortic sinus, supplies the right atrium, right ventricle, and posterior walls of the left ventricle
• Coronary veins: drain deoxygenated blood from the heart muscle (myocardium)
  • Great cardiac vein: runs alongside the LAD artery
  • Middle cardiac vein: runs in the posterior interventricular sulcus
  • Small cardiac vein: runs along the right coronary artery
  • Coronary sinus: receives blood from the cardiac veins and empties into the right atrium

Blood flow through heart chambers

1. Deoxygenated blood enters the right atrium via the superior and inferior vena cava
2. Blood flows through the tricuspid valve into the right ventricle
3. The right ventricle pumps blood through the pulmonary valve into the pulmonary trunk and arteries, which carry it to the lungs for oxygenation
4. Oxygenated blood returns from the lungs via the pulmonary veins and enters the left atrium
5. Blood flows through the mitral valve into the left ventricle
6. The left ventricle pumps oxygenated blood through the aortic valve into the aorta, which distributes it to the systemic circulation (body)

Cardiac function and regulation

• Cardiac cycle: the sequence of events that occur during one heartbeat, including systole (contraction) and diastole (relaxation)
• Conduction system: specialized cardiac tissues that generate and conduct electrical impulses to coordinate heart contractions
• Cardiac output: the volume of blood pumped by the heart per minute, calculated by multiplying heart rate and stroke volume
• Stroke volume: the amount of blood ejected from the left ventricle during one contraction